1. Field of the Invention
The present invention relates to a flexible printed circuit board.
2. Description of the Background Art
A flexible printed circuit (hereinafter referred to as FPC) board is used as a transmission path of digital signals between a central processing unit (hereinafter referred to as CPU) and a liquid crystal display in information communications equipment such as a cellular telephone. Prime constituents of the FPC board include a base insulating layer and conductive wiring traces formed on the insulating layer.
In recent years, higher frequency of the digital signals to be used has been realized with an increased amount of information transmitted through the information communications equipment. In the FPC board, however, transmission of the digital signals in a high-frequency region may cause high-frequency noise resulting from the harmonic of the digital signals to be generated from the wiring traces.
In this case, the high-frequency noise generated from the FPC board leads to malfunctions of other electronic components in some cases. In a cellular telephone including a highly sensitive antenna, for example, the high-frequency noise generated from the FPC board may be received by the antenna. Such a phenomenon causes malfunctions.
Therefore, an FPC board that suppresses external emission of the high-frequency noise generated from the wiring traces has been proposed (see JP 2008-300803 A, for example).
Conductive traces are formed on an insulating base film, and an insulating layer is formed on the base film to cover the conductive traces in the FPC board of JP 2008-300803 A. In addition, a conductive layer is formed on the insulating layer. In this case, the conductive layer on the insulating layer is used as a shield material of electromagnetic waves. This suppresses external emission of electromagnetic wave noise generated from the conductive traces.
Sufficient flexibility of the FPC board is desired in consideration of reduction in size and increased variety of the structure of the information communications equipment.
In the FPC board of JP 2008-300803 A, however, the conductive layer is provided on the conductive traces with the insulating layer sandwiched therebetween as described above. This leads to lower flexibility than that of an FPC board with no conductive layer formed thereon.